The present invention relates to an injector diagnosis system to diagnose an injector of a vehicle which is controlled by an electronic control unit (hereinafter abbreviated in an ECU) mounted on the vehicle.
In an air-fuel ratio control system, the ECU comprising a microcomputer which calculates a fuel injection quantity corresponding to an intake air quantity to supply a control signal to the injector so as to control the air-fuel ratio.
The control signal to the injector is usually a pulse signal synchronized with the rotation of an engine. By controlling the fuel injection time of the injector, that is, the width of the pulse signal, the air-fuel ratio is controlled.
Accordingly, when the injector malfunctions, the fuel quantity matching the intake air quantity is not injected. This causes the air-fuel ratio to be lean or rich. As a result, the exhaust emission is deteriorated, while engine output and fuel consumption are lowered.
For that reason, the ECU has recently been provided with the function to diagnose if an electric circuit having loads such as actuators normally operates in dependency on the control signal.
For example, Japanese Patent Laid-Open No. 63-27769 (1988) discloses the technology to detect the current flowing through the power line of a plurality of electric circuits, by means of a current sensor which is composed of a shunt and provided in the line so as to detect if the object to be controlled is in the normal operating state by verifying the detection signal of the current sensor with the control signal at that time by means of a detection circuit.
However, in the case of deceleration or engine overrunning, the fuel is cut off to the injector for fuel saving or protection of a catalytic convertor or the engine. When the fuel is cut off, a narrow width-pulse signal is generated to make the fuel injection time (or injection quantity) be zero or minimize it. Accordingly, the load current of the injector is made zero or minimized.
This means that the pulse signal is generated to detect the load current of the injector, even if the fuel is cut off. The judgement is thus erroneously made as if an abnormal current is supplied to the injector.